--- ---

WPF - Combobox with a null item

It adds an item for “Select All” or “Empty” functionality into a combobox

1. Converter

      public class ComboBoxEmptyItemConverter : IValueConverter
      {
          /// <summary> 
          /// this object is the empty item in the combobox. A dynamic object that 
          /// returns null for all property request. 
          /// </summary> 
          private class EmptyItem : DynamicObject	
      {

          public string Name { get; set; } = "Empty";
          public override bool TryGetMember(GetMemberBinder binder, out object result)
          {
              // just set the result to null and return true 
              result = null;
              return true;
          }
      }

      public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
      {
          // assume that the value at least inherits from IEnumerable 
          // otherwise we cannot use it. 
          IEnumerable container = value as IEnumerable;

          if (container != null)
          {
              // everything inherits from object, so we can safely create a generic IEnumerable 
              IEnumerable<object> genericContainer = container.OfType<object>();
              // create an array with a single EmptyItem object that serves to show en empty line 
              IEnumerable<object> emptyItem = new object[] { new EmptyItem() };
              // use Linq to concatenate the two enumerable 
              return emptyItem.Concat(genericContainer);
          }

          return value;
      }

      public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
      {
          return value is EmptyItem ? null : value;
      }
  }

3. XAML

    <StackPanel>
        <ComboBox ItemsSource="{Binding  Items, Converter={StaticResource ComboBoxEmptyItemConverter}}" 
              SelectedItem="{Binding SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource ComboBoxEmptyItemConverter}}"       
              DisplayMemberPath="Name" />
        <TextBlock Text="{Binding SelectedItem.Name}"/>
    </StackPanel>
   

   • https://github.com/atorres16/wpf-dropdown-null-item
   • https://stackoverflow.com/a/17903258/3596441
   • http://remyblok.tweakblogs.net/blog/7237/wpf-combo-box-with-empty-item-using-net-4-dynamic-objects.html